Cant do any of this one
Your text must have a definition of MRTS. Please provide that definition and then demonstrate how to use it.
Find the Partial Derivative and plug them in. Just keep track of all the other parameters floating around. They are to be treated as any other constant while finding the partial derivative.
Hmpf... "A" just looks like some constant that must be supplied by the problem statement, estimated with empirical data, or somehow pulled out of a hat.
Okay, one tiny hint. The MRTS refers to exchange of inputs to achieve CONSTANT production. It seems if we write the production equation as shown above, or if we divide by "A", it makes no difference to the MRTS. Thus, it is good that it is suggested to show the MRTS to be independent of "A".